The present invention relates to an electron cyclotron resonance negative ion source. It is advantageously applied in the production of high intensity H.sup.- ion beams (above 1 A) or the D.sup.- or T.sup.- isotopes thereof, said beams mainly being used for producing high energy neutral atom beams (intensity of several dozen amperes and energy of 200 to 500 KeV), which are more particularly used as effective heating means for thermonuclear plasmas produced in magnetic confinement fusion means. Moreover, these high intensity H.sup.-, D.sup.- or T.sup.- ion beams can be used in nuclear physics and in particular in tandem van der Graaf accelerators, or in the medical field using accelerators of the variable energy cyclotron type.
One of the presently used methods for producing negative ion beams and in particular H.sup.-, D.sup.- and T.sup.- ions is volume ionization. This is based on the formation, from a gas or a vapor contained in a closed enclosure, of a plasma mainly constituted in the case of hydrogen by H.sup.- and H.sup.+ ions and electrons.
This method firstly consists of producing molecules of hydrogen, deuterium or tritium, as a function of the starting gas used, which are vibrationally excited by hot or high energy electrons, i.e. having a kinetic energy above 20 eV, in accordance with the following reaction diagram (1) in the case of hydrogen: EQU e.sup.- +H.sub.2 .fwdarw.e.sup.- +H.sub.2 * (1)
Then, from the said (H.sub.2) excited molecules are formed H.sup.-, D.sup.- or T.sup.- ions by the following dissociative attachment reaction (2) in the case of hydrogen: EQU e.sup.- +H.sub.2 *.fwdarw.H.sub.2.sup.- .fwdarw.H.sup.- +H (2)
In this reaction diagram, the intermediate compound is unstable. The effective attachment cross-sections are high for co-called electrons having a kinetic energy at the most equal to 1 eV. This dissociative attachment phenomenon has in particular been described in an article by M. BACAL et al, Phys. Rev. Letters, 42, 1538, 1979.
The difficulty in such an enclosure of producing negative ions is linked with the production in the closed enclosure of the ion source a population of high energy or hot electrons and a population of cold electrons, which are spatially separated in such a way that the hot electrons do not destroy the negative ions formed by a collision which, in the case of hydrogen, is of the type: EQU H.sup.- +e.sup.- .fwdarw.H+2e.sup.- ( 3)
However, in the known negative ion sources functioning on the aforementioned principle, the destruction of the negative ions formed by reaction with the hot electrons of the plasma is relatively significant, which is prejudicial to the production of an intense negative ion beam. Generally, the number of negative ions constituting the plasma produced in the enclosure only represents 10% of the number of positive ions.
Moreover, in negative ion sources produced from a plasma, there is another problem linked with the method of extracting the negative ions by the electrostatic or ambipolar effect. Thus, the extraction or discharge by electrostatic effect of particles (positive ions, electrons, etc.) in a random particle source is always carried out by means of extraction electrodes raised to a positive potential compared with the walls of the enclosure formed, which is due to the high mobility of the plasma electrons. However, although for the extraction of positive ions, said positive potential aids the extraction, in the case of negative ions, said potential prevents the negative ions from leaving and electrostatically confines them in the enclosure. This is prejudicial to the production of an intense negative ion beam.